Programmed Death 1 (PD-1) (also known as Programmed Cell Death 1) is a type I transmembrane protein of 268 amino acids originally identified by subtractive hybridization of a mouse T cell line undergoing apoptosis (Ishida et al., Embo J., 11: 3887-95 (1992)). PD-1 is a member of the CD28/CTLA-4 family of T-cell regulators, and is expressed on activated T-cells, B-cells, and myeloid lineage cells (Greenwald et al., Annu. Rev. Immunol., 23: 515-548 (2005); and Sharpe et al., Nat. Immunol., 8: 239-245 (2007)).
Two ligands for PD-1 have been identified, PD ligand 1 (PD-L1) and PD ligand 2 (PD-L2), both of which belong to the B7 protein superfamily (Greenwald et al., supra). PD-L1 is expressed in a variety of cell types, including cells of the lung, heart, thymus, spleen, and kidney (see, e.g., Freeman et al., J. Exp. Med., 192(7): 1027-1034 (2000); and Yamazaki et al., J. Immunol., 169(10): 5538-5545 (2002)). PD-L1 expression is upregulated on macrophages and dendritic cells (DCs) in response to lipopolysaccharide (LPS) and GM-CSF treatment, and on T-cells and B-cells upon signaling via T-cell and B-cell receptors. PD-L1 also is expressed in a variety of murine tumor cell lines (see, e.g., Iwai et al., Proc. Natl. Acad. Sci. USA, 99(19): 12293-12297 (2002); and Blank et al., Cancer Res., 64(3): 1140-1145 (2004)). In contrast, PD-L2 exhibits a more restricted expression pattern and is expressed primarily by antigen presenting cells (e.g., dendritic cells and macrophages), and some tumor cell lines (see, e.g., Latchman et al., Nat. Immunol., 2(3): 261-238 (2001)). High PD-L1 expression in tumors, whether on the tumor cell, stroma, or other cells within the tumor microenvironment, correlates with poor clinical prognosis, presumably by inhibiting effector T cells and upregulating regulatory T cells (Treg) in the tumor.
PD-1 negatively regulates T-cell activation, and this inhibitory function is linked to an immunoreceptor tyrosine-based switch motif (ITSM) in the cytoplasmic domain (see, e.g., Greenwald et al., supra; and Parry et al., Mol. Cell. Biol., 25: 9543-9553 (2005)). PD-1 deficiency can lead to autoimmunity. For example, C57BL/6 PD-1 knockout mice have been shown to develop a lupus-like syndrome (see, e.g., Nishimura et al., Immunity, 11: 141-1151 (1999)). In humans, a single nucleotide polymorphism in the PD-1 gene is associated with higher incidences of systemic lupus erythematosus, type 1 diabetes, rheumatoid arthritis, and progression of multiple sclerosis (see, e.g., Nielsen et al., Tissue Antigens, 62(6): 492-497 (2003); Bertsias et al., Arthritis Rheum., 60(1): 207-218 (2009); Ni et al., Hum. Genet., 121(2): 223-232 (2007); Tahoori et al., Clin. Exp. Rheumatol., 29(5): 763-767 (2011); and Kroner et al., Ann. Neurol., 58(1): 50-57 (2005)). Abnormal PD-1 expression also has been implicated in T-cell dysfunctions in several pathologies, such as tumor immune evasion and chronic viral infections (see, e.g., Barber et al., Nature, 439: 682-687 (2006); and Sharpe et al., supra).
Recent studies demonstrate that T-cell suppression induced by PD-1 also plays a role in the suppression of anti-tumor immunity. For example, PD-L1 is expressed on a variety of human and mouse tumors, and binding of PD-1 to PD-L1 on tumors results in T-cell suppression and tumor immune evasion and protection (Dong et al., Nat. Med., 8: 793-800 (2002)). Expression of PD-L1 by tumor cells has been directly associated with their resistance to lysis by anti-tumor T-cells in vitro (Dong et al., supra; and Blank et al., Cancer Res., 64: 1140-1145 (2004)). PD-1 knockout mice are resistant to tumor challenge (Iwai et al., Int. Immunol., 17: 133-144 (2005)), and T-cells from PD-1 knockout mice are highly effective in tumor rejection when adoptively transferred to tumor-bearing mice (Blank et al., supra). Blocking PD-1 inhibitory signals using a monoclonal antibody can potentiate host anti-tumor immunity in mice (Iwai et al., supra; and Hirano et al., Cancer Res., 65: 1089-1096 (2005)), and high levels of PD-L1 expression in tumors are associated with poor prognosis for many human cancer types (Hamanishi et al., Proc. Natl. Acad. Sci. USA, 104: 3360-335 (2007), Brown et al., J. Immunol., 170: 1257-1266 (2003); and Flies et al., Yale Journal of Biology and Medicine, 84(4): 409-421 (2011)).
In view of the foregoing, strategies for inhibiting PD-1 activity to treat various types of cancer and for immunopotentiation (e.g., to treat infectious diseases) have been developed (see, e.g., Ascierto et al., Clin. Cancer. Res., 19(5): 1009-1020 (2013)). In this respect, monoclonal antibodies targeting PD-1 have been developed for the treatment of cancer (see, e.g., Weber, Semin. Oncol., 37(5): 430-4309 (2010); and Tang et al., Current Oncology Reports, 15(2): 98-104 (2013)). For example, nivolumab (also known as BMS-936558) produced complete or partial responses in non-small-cell lung cancer, melanoma, and renal-cell cancer in a Phase I clinical trial (see, e.g., Topalian, New England J. Med., 366: 2443-2454 (2012)), and is currently in Phase III clinical trials. MK-3575 is a humanized monoclonal antibody directed against PD-1 that has shown evidence of antitumor activity in Phase I clinical trials (see, e.g., Patnaik et al., 2012 American Society of Clinical Oncology (ASCO) Annual Meeting, Abstract #2512). In addition, recent evidence suggests that therapies which target PD-1 may enhance immune responses against pathogens, such as HIV (see, e.g., Porichis et al., Curr. HIV/AIDS Rep., 9(1): 81-90 (2012)). Despite these advances, however, the efficacy of these potential therapies in humans may be limited.
Therefore, there is a need for a PD-1-binding agent (e.g., an antibody) that binds PD-1 with high affinity and effectively neutralizes PD-1 activity. The invention provides such PD-1-binding agents.